Field of the Invention
The present disclosure relates to an organic light emitting diode display device and, more particularly, to an organic light emitting diode display device having a microcavity effect.
Discussion of the Related Art
Among various flat panel displays (FPDs), an organic light emitting diode (OLED) display device has superior properties such as high luminance and low driving voltage. The OLED display device uses an emissive electroluminescent layer to realize a high contrast ratio and a thin profile, and is excellent at displaying a moving image because of a short response time of several micro seconds (μsec). Also, the OLED display device has no limitation on a viewing angle and is stable even in a low temperature. Since the OLED display device is typically driven by a low voltage of about 5V to about 15V in direct current (DC), fabrication and design of a driving circuit is easy. Accordingly, the OLED display device has been used for various information technology (IT) devices, such as televisions, monitors and portable phones.
In general, an OLED display device includes an array element and a light emitting diode. The array element includes a switching thin film transistor (TFT) connected to a gate line and a data line and a driving TFT connected to the light emitting diode. The light emitting diode includes a first electrode connected to the driving TFT, an emitting layer, and a second electrode.
The light generated in the emitting layer is emitted through the first electrode or the second electrode to display an image. Recently, a top emission type OLED display device where the light is emitted through the second electrode has been widely used in consideration of an aperture ratio. In the top emission type OLED display device, the distances between the first and second electrodes are different from each other in red, green and blue pixel regions for improving a color purity of red, green, and blue and for increasing emission efficiency due to a microcavity effect.
The emitting layer may be formed by a vacuum thermal evaporation method using a shadow mask. However, as a size of the display device increases, a sagging occurs in the shadow mask and deterioration in evaporation increases. As a result, it becomes more difficult to apply the vacuum thermal evaporation method to the large-sized substrate. In addition, because a shadow effect occurs in the vacuum thermal evaporation method, it is difficult to fabrication of the OLED display device having a high resolution over 250 PPI (pixel per inch) is more difficult by the current technology.
Further, because the thermal evaporation method using the shadow mask is performed in a vacuum state, a vacuum chamber is required for obtaining the vacuum state. In addition, since an additional process and additional time are required for changing the inside of the vacuum chamber from an atmospheric pressure state to a vacuum state, productivity per hour decreases and fabrication cost increases.
As a result, an inkjet method of forming the emitting layer substituted for the vacuum thermal evaporation using the shadow mask has been suggested. In the inkjet method, after an inkjet apparatus jets an emitting material of a liquid phase inside a bank layer, the emitting material is cured. Since the emitting layer is selectively formed in a predetermined region by the inkjet apparatus, waste of a material is prevented. Further, the inkjet apparatus has an advantage in maintenance and management.
However, since an amount of an emitting material jetted by a single drop is determined due to limitations of the inkjet apparatus, the emitting layer having a thickness for the microcavity effect is not formed by the single drop in the inkjet apparatus. Accordingly, the emitting layer is formed by multiple drops over three repetitions. After the emitting material is jetted by a drop, a drying step for the jetted emitting material is performed for a long time period over several tens of minutes before the process can be repeated for the next drop. As the number of drops of the emitting material increases, productivity per hour decreases.